1. Field of the Invention
The present invention relates to a super-resolution optical recording medium which can reproduce information by irradiating reproduction light on recording marks formed on a recording layer. In particular, the present invention relates to the super-resolution optical recording medium from which small recording marks the size of which is equal to, or less than, the resolution limit of a reproduction optical system can be reproduced, and a method for recording information on the super-resolution optical recording medium.
2. Description of the Related Art
In recent years, as described in, for example, Japanese Patent Laid-Open Publication No. 2003-6872, a super-resolution optical recording medium has been proposed, from which a recording mark train smaller than the diffraction limit of a reproduction optical system can be reproduced.
In the case of a conventional optical recording medium, in general, it is impossible to read a recording mark row the period of which is a certain recording mark row period or less by a reproduction method using light. The length of this recording mark row period is referred to as a diffraction limit. In a reproduction optical system with a wavelength of λ and a numerical aperture of NA, the diffraction limit is represented as λ/NA/2. If the length of a recording mark is equal to that of a blank in one period, the length of the recording mark is represented as λ/NA/4. The length of the recording mark is referred to as a resolution limit.
Thus, reducing the wavelength λ and/or increasing the numerical aperture NA reduces the resolution limit, and hence increases recording density. Further shortening the wavelength and increasing the numerical aperture are reaching their limits. The foregoing super-resolution optical recording medium adopts technologies for reproducing recording marks smaller than λ/NA/4 to further increase the recording density without shortening the wavelength λ and increasing the numerical aperture NA.
The above-described conventional optical recording medium includes a phase-change recording film, as described in, for example, Scanning Probe Microscope Observation of Recorded Masks in Phase Change Disks: Takashi Kikukawa and Hajime Utsunomiya, Microsc. Microanal., 7 (2001)363-367. Therefore, the recording mark or the vicinity thereof is not deformed by the recording operation.